The need for improved methods and materials to manage severe maxillofacial injuries is well recognized. In such cases, the surgeon faces the dual problem of restoring function and appearance. The patient suffering from an extensive maxillofacial injury is typically confronted with disfigurement, impaired speech, and eating difficulties as well as the psychological trauma resulting from the injury.
Ideally, fixation applicances should maintain the fractured bone segments in close approximation for the promotion of primary union and healing, provide sufficient strength and rigidity to prevent disruption of the primary union by external forces, and as the union becomes further ossified, transfer an increasing proportion of the external load to the healing bone so that it will be strained and exercised. The fulfillment of these criteria is necessary for the formation of healthy, hard tissue that has properties commensurate with those of virgin bone.
Implant materials used for such injuries over the years belong to three traditional classes: metals, ceramics, and polymers. The choice of material for the particular application depends on the type and magnitude of applied loads which the implant is expected to experience in vivo and whether the implant is to be a permanent or a temporary augmentation. When trying to make repairs to the skeletal system, surgeons and engineers must attempt to replicate the static and dynamic responses of bone. Bone consists of a framework of collagenous fibers, a mineral matrix consisting primarily of calcium hydroxyapatite, and a small amount of polysaccharides. Although bone is stronger and less deformable than polymeric materials, it is weaker than metals. Historically, metals have received wide application for the construction of devices for fixing fractures. Metals exhibit high values of tensile strength and compressive modulus; they can be fabricated into fixation hardware by a variety of conventional techniques; and they provide excellent resistance to the in vivo environment. Metals and alloys now used as surgical implants include 316 stainless steel, several cobalt-chromium alloys, titanium, zirconium alloys and tantalum.
In mandibular fracture repair, one of the major disadvantages with metal implants is atrophy of the healing bone as a result of the stress-protection effect of the rigid metal plate. Other drawbacks with metal fixation appliances are that they may cause local inflamation and may corrode with age.
Among the metallic materials, tantalum is superior in resistance to corrosion and has been extensively employed as fixation plates for fractured bones and as implants. The metal is, however, difficult to process. In contrast, ceramic materials show good affinity to bones often with bone tissue penetrating into the fine pores of the ceramic to produce a strong fixation. Bone and tissue compatibility with ceramics is excellent. The main disadvantage of ceramic materials is their poor impact strength as they are often brittle. This condition is quite evident with the more porous ceramics and leads to poor durability of ceramic implants and fixation devices. On the other hand, polymeric materials provide excellent impact strength, good biocompatibility, and they are easily molded to the desired shape; however, they do not possess the required strength and stiffness for bone fixation.
Those materials and many of the prior art materials suffer from the common drawback of being permanent. In many applications, such as a fixation appliance holding a fracture together while it heals, it is highly desirable if the implant can be resorbed by the body. Such an implant would biodegrade over a period of weeks or years, and be gradually replaced by natural bone growth. Such materials eliminate the need for a second surgery to remove the implant. However, homogenous fixation plates previously fabricated from biodegradable polymers have been shown to possess insufficient strength and rigidity for initial fracture fixation. Porous resorbable ceramics have also been used in bone repair, but they must be used in conjunction with other support because of their fragile nature.
The prior art includes U.S. Pat. No. 3,929,971 which discloses a synthetic material (either hydroxapatite or whitlockite) that may be used with other materials, such as organic polymers, to form a composite substance which could be useful in constructing a degradable prosthetic implant; U.S. Pat. No. 3,905,047 which discloses a biodegradable prosthesis containing an eutectic or metal pyrophosphate and high-modulus fibers formed of a refractory metal oxide; U.S. Pat. No. 4,330,514 is directed to a process for preparing a hydroxyapatite ceramic which is used in a nondegradable implant comprising the ceramic and an organic binding material; and U.S. Pat. No. 4,356,572 which is directed to a porous biodegradable bone implant which utilizes calcium carbonate in crystalline form.